1. Field of the Invention
The present invention generally relates to electronic devices, and, more specifically, to field effect transistors.
2. Description of the Related Art
Advanced radar and communication systems, as well as other electronics applications, depend on high-performance semiconductor amplifier devices. Limited performance of amplifier devices results in constrained performance of radar and communication systems in terms of high initial cost, undesirable failure rate, difficulty using higher frequency bands, and undesirably large volume, mass, power consumption and heat dissipation.
One type of field effect transistor that is used for high-frequency applications is a high electron mobility transistor (HEMT) that is able to operate at higher frequencies than an ordinary doped transistor. An HEMT uses a junction between two materials with different band gaps as the channel instead of a doped region as typically used in a conventional metal-oxide-semiconductor field-effect transistor (MOSFET). The bandgap difference results in the formation of conduction and valence band discontinuities at the layer interface creating a quantum well in the conduction band. The wider band gap semiconductor is doped with donors while the smaller band gap material is left undoped. The conduction band electrons move from the donor layer to the undoped layer, also called the transport layer, forming a two-dimensional electron gas (2DEG) in the transport layer along the interface. Since there are no impurities in the transport layer, there are no scattering centers. The presence of additional electrons in the transport layer provides higher conductivity without the scattering that slows down the electrons, thus providing much higher mobility that enables higher-frequency operation. A cross-section of a conventional HEMT is shown in FIG. 1.
In many applications, a conventional HEMT is biased into its linear region by combining a negative direct current (DC) bias voltage with the alternating current (AC) signal to drive the gate of the HEMT. In many situations, for example on a satellite, a dedicated negative power supply must be provided for this purpose, adding to the weight, cost, and complexity of the system.
Researchers have attempted to increase the gain of HEMTs by adding a second gate electrode beneath the channel. It has been demonstrated that this approach provides increased gain through the tremendous increase in transconductance. However, construction of such an HEMT is extraordinarily complex and is not amenable to manufacturing in useful numbers.